The present invention relates to an automatic focusing apparatus for use with optical instruments such as photographic cameras, television cameras and the like.
Various types of automatic focusing apparatus have been proposed, one of which is of the type having infrared-light-emitting means for illuminating a spot on the subject, wherein the infrared light reflected from the subject is sensed by a light-sensing means to determine automatically the distance from the apparatus to the subject to be photographed. As is well known, since this type of automatic focusing apparatus is based on a principal of triangulation, there is the disadvantage that an increased base length (the distance between the infrared-light-emitting means and the reflected light sensing means) is needed in order to measure the distance with a high accuracy. As a result, such automatic focusing apparatus tends to be of increased size.
An automatic apparatus which is proposed to eliminate the above-mentioned disadvantage is described in Japanese Open Pat. No. 54-155832 wherein the infrared light is directed to the subject through a picture-taking lens and the reflected infrared light from the subject is transmitted to an infrared sensing means through the same lens. In such apparatus, because the light sensing means receives marginal rays passing through the picture-taking lens, the center of the flux is coincident with that of the infrared sensing means when the lens is properly focused and the former is spaced apart from the latter when the lens is out of focus. However, it is necessary, in this automatic focusing apparatus, to detect a peak output from the light sensing means, by providing a sample hold circuit, a peak value holding circuit and the like. As a result of this, the latter automatic focusing apparatus is complex in construction.